Method of seamless roaming between wireless local area networks and cellular carrier networks

ABSTRACT

A scheme that enables seamless roaming between the WLAN and the cellular carrier network by enabling a user that originates a call in the WLAN and happens to go outside the range of the WLAN to automatically switch over to the cellular carrier network without losing connection with the other party. This solution assumes that the mobile device has the capability to operate in at least two modes that include the WLAN mode and one of the cellular carrier modes, such as the GSM, IS-95 CDMA, IS-136 TDMA, and iDEN.

RELATED APPLICATIONS

This application claims the benefit of provisional application Ser. No.60/419,674 filed on Oct. 18, 2002 which is incorporated by referenceherein in its entirety.

BACKGROUND OF THE INVENTION

The proliferation of the wireless local area networks (WLANs) has led tothe search for ways in which its utilization can be increased. By WLAN,we include all instantiations of such technologies as 802.11a, 802.11b,802.11g, Bluetoooth and any similar WLAN versions. For example, thespecification for the IEEE 802.11 version of WLAN includes an accessscheme called the distributed control function, which permits thenetwork to support both data and voice applications. Today voice overwireless local area network (VoWLAN) is a reality. The voice may beencoded and transmitted using voice over internet protocol (VoIP) formatand protocols such as G.711, G.726, G.729, SIP, MEGACO, H.323, or othersimilar protocols that are being developed.

One of the issues in WLAN is that there is a limited range of operationdue to power requirements. For example, a typical IEEE 802.11 WLAN has arange of at most 300 yards from the access point that connects themobile devices to the wired LAN. Thus, when an IEEE 802.11-based mobiledevice roams beyond this range, any call in progress is forciblyterminated.

SUMMARY OF THE INVENTION

The present invention concerns a scheme that enables seamless roamingbetween the WLAN and the cellular carrier network. The method enables auser that originates a call in the WLAN and happens to go outside therange of the WLAN to automatically switch over to the cellular carriernetwork without losing connection with the other party. This solutionassumes that the mobile device has the capability to operate in at leasttwo modes that include the WLAN mode and one of the cellular carriermodes, such as the GSM, IS-95 CDMA, IS-136 TDMA, and iDEN.

The above and other features of the invention including various noveldetails of construction and combinations of parts, and other advantages,will now be more particularly described with reference to theaccompanying drawings and pointed out in the claims. It will beunderstood that the particular method and device embodying the inventionare shown by way of illustration and not as a limitation of theinvention. The principles and features of this invention may be employedin various and numerous embodiments without departing from the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, reference characters refer to the sameparts throughout the different views. The drawings are not necessarilyto scale; emphasis has instead been placed upon illustrating theprinciples of the invention. Of the drawings:

FIG. 1 is a block diagram showing the architecture of a typical WLANthat supports Voice communications;

FIG. 2 is a timing diagram showing the initial mobile deviceregistration process;

FIG. 3 is a timing diagram showing the handoff procedure for a mobiledevice with a PSTN-based call in progress;

FIG. 4 is a timing diagram showing the handoff procedure for a mobiledevice with cellular network-based call in progress; and

FIG. 5 is a timing diagram showing the procedure for cellularnetwork-to-WLAN handoff.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A typical WLAN is comprised of access points (APs) that are connected tothe enterprise LAN via an access controller (AC) or a wireless LANswitch. The access controller/wireless LAN switch AC is the center ofintelligence of the WLAN and is responsible for admission control,authentication and mobile device roaming coordination. One AC/wirelessLAN switch can control several APs and multiple ACs/wireless LANswitches can be in the same network. Another device called the CellularProxy (CP) is a gateway that connects the enterprise LAN to the cellularnetwork. If the enterprise PBX is not VoIP-capable, the cellular proxyalso provides the VoIP interface between the enterprise LAN and the PBX.The Cellular Proxy is located in the part of the enterprise buildingthat has a very good cellular coverage.

The Cellular Proxy is not necessarily tightly coupled to any cellularnetwork. To any cellular network, it is a bank of radios. It hides thedetails of the movement of the mobile devices within the enterprisepremises from the cellular network thereby preventing the cellularnetwork from making frequent updates to its database. Also, it canconnect to multiple cellular networks simultaneously because it containsradios for different types of cellular network technologies, such asCDMA, AMPS TDMA, GSM TDMA, iDEN, WCDMA, CDMA2000, GPRS, 1XRTT, 1xEVDO,and 1xEVDV. Thus, the Cellular Proxy can proxy for mobile devices inCDMA-based and TDMA-based cellular networks simultaneously.

Mobile devices, such as laptops that are equipped with wireless networkinterface cards and personal digital assistants access the network viathe APs. A typical network is shown in FIG. 1. In the figure, the accesscontroller and/or wireless LAN switch are not shown; they are assumed tobe part of the enterprise LAN infrastructure.

Each dual-mode mobile device MD has two telephone numbers: one number isassigned by the enterprise as an extension of the enterprise's PBX, andthe other number is assigned by the cellular carrier. The defaultnetwork for each mobile device is the enterprise (or hotspot) WLAN,which means that when a mobile device is turned on, it first searchesfor the enterprise's WLAN and registers with the network, if it isfound. If the enterprise WLAN is not found the mobile device MD thenregisters with the cellular carrier network. The rationale for thisoperational requirement is to save the enterprise money by ensuring thatmobile device users do not accrue air charges when they are within thecorporate WLAN. All calls generated by the mobile device while it is onthe enterprise network are routed via the corporate PBX to the publicswitched telephone network (PSTN).

Since each mobile device has two telephone numbers, it can be reached intwo ways: via the PBX when calls come from the PSTN, and via theCellular Proxy CP when calls come from a cellular network. Regardless ofhow the calls arrive, they are converted into voice over IP (VoIP)packets by the appropriate device and presented to the mobile device viathe WLAN. These calls are based on the Session Initiation Protocol(SIP), which has the advantage over the ITU-T H.323 protocol in that itis a lightweight protocol that leverages the Internet protocols.

Each mobile device MD goes through an initialization process when it isturned on in the WLAN. As discussed earlier, the default network is theenterprise WLAN. Therefore, when a mobile device is turned on it sends aRegistration Request message to the appropriate authentication server inthe enterprise LAN. After the authority server has authenticated themobile device, it returns a Registration Complete message to the device.The authentication server, which has information on each mobile device'scellular service provider's network, then sends a Registration Requestmessage to the Cellular Proxy instructing the latter to register themobile device in the device's cellular carrier network. The CellularProxy CP first sets up a TCP connection to the mobile device MD via theappropriate access controller (or wireless LAN switch) and access pointbefore commencing the registration of the device in the cellularnetwork. After the Cellular Proxy has successfully registered the mobiledevice in the cellular network, it returns a Registration Completemessage to the authentication server. The Cellular Proxy then startslistening on the appropriate paging channel for calls destined for themobile device from the cellular carrier network and will deliver suchcalls to the mobile device via the appropriate access controller (or thewireless LAN switch) and access point. The message flow for theregistration process is illustrated in FIG. 2.

Consider a dual-mode mobile device that originates a call within a WLAN.As the user moves closer to the edge of the network the signal qualitybegins to degrade. The degradation will reach a point where the signalstrength is almost imperceptible, which causes the call to beterminated.

Here, the mobile device MD has the capability to monitor the signalquality by measuring the signal-to-noise ratio (SNR). Assume also thatfrom practical experience acquired through measurements it is known thatwhen SNR reaches some threshold value d, the voice quality becomesunacceptable. The goal is to prevent the call quality from degrading tothis critical point. Thus, when the SNR drops to a cutoff value r>d, thesystem initiates a handoff with the objective of completing the handoffprocedure before the SNR drops down to the threshold value d. Thus thescheme operates in the following manner:

When a mobile device experiences SNR measurement value of r, it sends aHandoff Request message to the Cellular Proxy via the TCP connectionthat exists between the two devices.

When Cellular Proxy receives the message it takes one of two actionsthat depend on where the other party in the call is located.

If the other party is located in the PSTN, which means that the callpasses through the PBX, then it takes the following actions: 1) theCellular Proxy uses one of its own carrier-assigned telephone numbers tocall the mobile device's carrier-assigned telephone number; 2) since themobile device is not physically connected to the cellular network, theCellular Proxy will also receive the call on behalf of the device; 3)after receiving the parameters of the call from the cellular network,such as the channel or code to use, power level, etc., the CellularProxy will forward these parameters to the mobile device over the TCPconnection that it established between the two and commands the mobiledevice to switch its radio to the cellular network using thoseparameters; 4) the Cellular Proxy will then close the TCP connection,stop proxying for that device in the cellular network to avoid cloningproblems, and will thereafter forward the call to the mobile device overthe new connection established via the cellular network; and 5) onreceiving the call parameters, the mobile device will immediately switchits radio to the cellular network without having to register again sinceit has already been registered and authenticated in the cellularcontroller by the Cellular Proxy. As it moves from base station to basestation outside the enterprise network, the mobile device will besubject to the handoff mechanism that applies within the cellularnetwork.

This sequence of activities is illustrated in FIG. 3.

If the other party is located in the cellular carrier network, whichmeans that the call passes through the Cellular Proxy without reachingthe PBX, it takes the following set of actions: 1) since the CellularProxy has all the parameters of the call, it returns these parameters tothe mobile device via the TCP connection between the two and commandsthe mobile device to switch its radio to the cellular network usingthose parameters; 2) the Cellular Proxy will then close the TCPconnection and stop proxying for the mobile device to avoid creatingcloning problems in the cellular network; 3) Upon receiving theinformation, the mobile device will tune its radio to the channelcurrently used by the Cellular Proxy and receive the call directly fromthe cellular network.

This sequence of activities is illustrated in FIG. 4.

Assume that the mobile device has a call in progress as it is moved fromthe outside world into the enterprise premises. The operational rule isthat if there is good cellular coverage within the enterprise premises,the call will be allowed to complete in the cellular network after whichthe mobile device will register in the enterprise LAN using theprocedure described earlier. However, if there is no good coveragewithin the enterprise premises, the device will initiate a handoff withthe Cellular Proxy. The procedure is as follows:

1) if the SNR reaches the predefined cutoff value, the mobile devicesends a short message service (SMS) message to the Cellular Proxy. Themessage contains information on the identity of the mobile device, suchas its PBX extension and its cellular network telephone number as wellas the parameters of the current call. The Cellular Proxy maintains arecord of the cellular network that each enterprise mobile device isassociated with. Therefore, with the information it received from themobile device the Cellular Proxy sends a message to the authenticationserver to expedite the authentication of the mobile device.

2) The authentication server will provide emergency registration for themobile device by broadcasting a Registration Invite message that themobile device will respond to.

3) After locating and authenticating the mobile device, theauthentication server forwards the device's location and networkconfiguration parameters like the IP address to the Cellular Proxy.

4) After sending the emergency registration request to theauthentication server, the Cellular Proxy will start monitoring thechannel on which the mobile device was communicating and accumulatinginformation destined for the device until the device has beenauthenticated and registered in the network

5) When the Cellular Proxy receives information on the device'slocation, it will set up a TCP connection to the device and forward allaccumulated packets to the device.

6) After this, the operation becomes similar to that described earlier.The Cellular Proxy listens on the channel and relays information betweenthe mobile device and the cellular network until the conversation isover and the connection is terminated. When the current call ends, theCellular Proxy continues to listen on the cellular network's pagingchannel for calls destined for the mobile device, as described earlier.

This sequence of activities is illustrated in FIG. 5.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1. A method of handling calls between a wireless local area network ofan enterprise and a cellular carrier network for a mobile terminal thatis capable of communicating over the wireless local area network and thecellular carrier network, the method comprising: registering andemulating the mobile terminal on the cellular carrier network when themobile terminal is communicating via the wireless local area networkwithin the enterprise; receiving, via a fixed radio terminal, calls fromthe cellular carrier network; and routing, via the fixed radio terminal,the received calls received from the cellular carrier network to themobile terminal via the wireless local area network, wherein routing thereceived calls comprises calling a telephone number of the mobileterminal for a call of the received calls involving a party in a givennetwork.
 2. A method as claimed in claim 1, further comprising:inheriting call parameters; and switching a radio to the cellularcarrier network using the call parameters responsive to the mobileterminal moving from the wireless local area network into the cellularcarrier network.
 3. A method as claimed in claim 1, wherein the mobileterminal is configured to be assigned a telephone number for thecellular carrier network and a telephone number for a private branchexchange of the enterprise.
 4. A method as claimed in claim 3, furthercomprising: for calls placed to the telephone number for the cellularcarrier network, receiving the calls at the fixed radio terminal androuting the calls to the mobile terminal via the wireless local areanetwork when the mobile terminal is on the wireless local area network;and for calls placed to the telephone number for the private branchexchange, receiving the calls and routing the calls to the fixed radioterminal via the wireless local area network when the mobile terminal ison the wireless local area network.
 5. A method as claimed in claim 1,wherein registering and emulating the mobile terminal comprises:receiving a registration request on behalf of the mobile terminal;responsively registering the mobile terminal on the cellular carriernetwork; and responsive to successful registration of the mobileterminal on the cellular carrier network, sending a registrationcomplete message.
 6. A method as claimed in claim 1, further comprisingmonitoring a call for call quality over the wireless local area network.7. A method as claimed in claim 6, further comprising, in response todetermining that the call quality has degraded to a threshold, switchingto the cellular carrier network.
 8. A method as claimed in claim 6,further comprising: responsive to determining the call passes through aprivate branch exchange of the enterprise, calling a telephone number ofthe mobile terminal on the cellular carrier network and routing the callto the mobile terminal through the cellular carrier network.
 9. A methodas claimed in claim 6, further comprising: responsive to determining thecall is on a phone number of the cellular carrier network when themobile terminal is switching to the cellular carrier network, handingoff the call to the mobile terminal.
 10. A system for managing callsbetween a wireless local area network (wireless LAN) and a cellularcarrier network, the system comprising: a fixed radio terminalconfigured to communicate with the cellular carrier network and toemulate a mobile terminal on the cellular carrier network; and a gatewayconfigured to: receive a registration message from the mobile terminalvia the wireless LAN, register the mobile terminal on the cellularcarrier network via the fixed radio terminal, receive a call from thecellular carrier network via the fixed radio terminal, and route thereceived call to the mobile terminal over the wireless local areanetwork by calling a telephone number of the mobile terminal when thereceived call involves a party in a given network.
 11. A system asclaimed in claim 10, wherein the gateway is further configured to:receive an indication of a handoff; send call parameters to the mobileterminal; and instruct the mobile terminal to switch a radio to thecellular carrier network using the call parameters.
 12. A system asclaimed in claim 10, wherein the gateway is further configured toperform TDMA-to-VoIP conversion.
 13. A system as claimed in claim 10,wherein the mobile terminal is configured to be assigned at least twotelephone numbers, comprising a telephone number for the cellularcarrier network and a telephone number for a private branch exchange.14. A system as claimed in claim 13, wherein the gateway is furtherconfigured to: receive calls placed to either the telephone number forthe cellular carrier network or to the telephone number for the privatebranch exchange and route the received calls to the mobile terminal viathe wireless LAN when the mobile terminal is on the wireless LAN.
 15. Asystem as claimed in claim 10, wherein the mobile terminal is furtherconfigured to: attempt to register with the wireless LAN; and registerwith the cellular carrier network when registration with the wirelessLAN is unsuccessful.
 16. A system as claimed in claim 10, wherein themobile terminal is configured to monitor one or more calls for callquality over the wireless LAN.
 17. A system as claimed in claim 16,wherein the mobile terminal is further configured to switch tocommunicating over the cellular carrier network in response todetermining that the call quality of a received call has degraded belowa threshold value.
 18. A system as claimed in claim 17, wherein thegiven network comprises the public switched telephone network (PSTN).19. A system as claimed in claim 18, wherein the gateway is furtherconfigured to receive a call placed to the telephone number of thecellular carrier network.
 20. A system as claimed in claim 19, whereinthe gateway is further configured to: receive parameters of the callplaced to the telephone number of the cellular carrier network; andforward the parameters to the mobile terminal.
 21. A system as claimedin claim 10, wherein the mobile terminal is configured to monitor one ormore calls for call quality over the cellular carrier network.
 22. Asystem as claimed in claim 21, wherein the mobile terminal is furtherconfigured to switch to communicating over the cellular carrier networkwhen the call quality of a received call from the cellular carriernetwork degrades below a threshold value.
 23. A system as claimed inclaim 22, wherein the gateway is further configured to: determinewhether a call quality of a given call has degraded to the threshold;and responsive to determining the call quality of the given call hasdegraded to the threshold, monitor communications for the mobileterminal on the cellular carrier network, and send communications to themobile terminal via the wireless LAN and communications from the mobileterminal to the cellular carrier network via a fixed antenna.
 24. Asystem as claimed in claim 10, wherein the gateway is further configuredto perform CDMA-to-VoIP conversion.
 25. A system for managing callsbetween a wireless local area network and a cellular carrier network,the system comprising: a mobile terminal capable of communicating overthe wireless local area network and the cellular carrier network; and agateway that registers and emulates the mobile terminal on the cellularcarrier network when the mobile terminal is communicating via thewireless local area network, the calls from the cellular carrier networkbeing received via a fixed radio terminal of the gateway, the gatewayconfigured to route calls received from the cellular carrier network tothe mobile terminal over the wireless local area network; wherein themobile terminal is configured to be assigned a telephone number for thecellular carrier network and a telephone number for a private branchexchange and calls placed to the telephone number of the cellularcarrier network are received by the fixed radio terminal of the gatewayand routed to the mobile terminal via the wireless local area networkwhen the mobile terminal is on the wireless local area network and callsplaced to the telephone number of the private branch exchange arereceived by the gateway and routed to the mobile terminal via thewireless local area network when the mobile terminal is on the wirelesslocal area network; wherein the mobile terminal attempts to registerwith the wireless local area network and only registers with thecellular carrier network if registration with the wireless local areanetwork is unsuccessful; and wherein if a call is placed to thetelephone number of the private branch exchange while the mobileterminal is communicating over the cellular carrier network, then thegateway calls the telephone number of the mobile terminal on thecellular carrier network and routes the call to the mobile terminalthrough the cellular carrier network.
 26. A method, comprising:registering, at a gateway, a mobile terminal via a wireless local areanetwork (LAN); establishing, at the gateway, a wireless-LAN connectionto the mobile terminal via the wireless LAN; establishing, at thegateway, a wireless connection with a cellular carrier network on behalfof the mobile terminal; receiving, at the gateway, a cellular callintended for the mobile terminal from the cellular carrier network androuting the call to the mobile terminal via the wireless-LAN connection;receiving, at the gateway, a handoff request from the mobile terminal;and responsive to the handoff request, the gateway: making a second callto a telephone number of the mobile terminal, and receiving the secondcall on behalf of the mobile terminal.
 27. The method of claim 26,further comprising: instructing the mobile terminal to switch a radio tothe cellular carrier network based on call parameters.
 28. The method ofclaim 26, further comprising: closing the wireless-LAN connection; andceasing communications on behalf of the mobile terminal in the cellularcarrier network.